A. Field of the Invention
This invention relates to a system for reciprocally rotating an output element. Specifically it relates to a two-degree-of-freedom system in which an alternating driven torque is applied to a relatively massive driven element such as a motor armature to provide a relatively large angular excursion in an output element elastically coupled to the driven element.
While the invention is useful in a number of applications it is directed specifically to an optical scanner. The scanner is used to sweep a light beam back and forth over an object and the reflected or transmitted light from the object is sensed to ascertain a characteristic of the object. For example, at the checkout counter in a retail store, a scanner can be used to sweep a light beam back and forth over a bar code imprinted on each package to provide information relating to the contents of the package. The light reflected from the bar code is sensed to ascertain the information contained in the code.
B. Prior Art
Ordinarily, an optical scanner is driven by means of a limited-rotation electric motor, for example a motor of the type used to drive a recording pen in a strip chart recorder. A mirror is affixed to the motor shaft and the motor is driven at a frequency corresponding to the desired scanning rate, so that the mirror reflects a light beam at the desired rate onto the object to be monitored.
A particularly desirable motor for this application is described in U.S. Pat. No. 3,177,385. In this motor the magnetic flux in the air gap is perpendicular to the direction of the armature movement. The motor is characterized by the combination of a relatively high efficiency with a relatively large angular excursion. On the other hand, the armature is subject to unbalancing magnetic forces that tend to tilt it or radially displace it into contact with the stator poles of the motor. These unbalancing forces are contained by ball bearings that support the armature shaft at opposite ends of the motor. However, ball bearings have a relatively short life span when subjected to rapid reciprocal motion, e.g. at cyclic rates of 400 Hz or more. Torsional armature supports are therefore sometimes substituted for ball bearings to avoid the problem of bearing wear. Indeed if it is not overstressed, a torsional support has an essentially unlimited life span.
But if a torsional support is stiff enough in the bending mode to prevent tilting or radial displacement of the armature in a perpendicular-flux motor, it will be so stiff in the torsional mode that it will be over-stressed when the armature is rotated through the angular excursions required for such applications as optical scanning. The support will therefore fail after a relatively short life span. The principal object of the present invention is to provide a reciprocating scanner that can include a perpendicular-flux motor and yet have an armature support characterized by a relatively long life span.